Manufacturers of exterior coatings, such as paints and other finishes, as well as plastics and other components which tend to degrade under exposure to solar radiation and other weathering effects, want to know how such products will perform following years of exposure. However, such manufacturers typically require this information in a much shorter time period than it would take to expose such materials to weathering effects under normal conditions. Accordingly, accelerated weathering apparatus have been developed which accelerate the effects of weathering due to outdoor exposure in a much shorter time so that manufacturers need not actually wait five or ten years in order to determine how their products will hold up after five or ten years of actual outdoor exposure.
Weathering is the adverse response of a material or product to its climate, often causing unwanted and premature product failures. Consumers spend millions of dollars per year to maintain products that inevitably degrade and to replace products that fail. Materials that fail as a result of exposure to outdoor environments account for a significant portion of this total cost.
The three main factors of weathering are solar radiation (light energy), temperature and water (moisture). But it is not just “how much” of each of these factors ultimately cause degradation to materials, because different types of solar radiation, different phases of moisture, and temperature cycling have a significant effect on materials on exposure. These factors, in conjunction with secondary effects such as airborne pollutants, biological phenomenon, and acid rain, act together to cause “weathering.”
One outdoor natural accelerated weathering test apparatus (“ONAWTA”) is disclosed in U.S. Pat. No. 4,807,247, issued to Robbins, III.
A specimen that is undergoing an outdoor accelerated weathering exposure test experiences high levels of natural ultraviolet irradiation and is expected to provide an accelerated indication of how the test specimen material will degrade under natural exposure to ultraviolet weathering. Photo-oxidation weathering reactions require oxygen. Surface layers of the test specimen material readily absorb oxygen from adjacent ambient air volume. However, important weathering reactions may also take place in the bulk or “non-surface areas” of thicker volumes of test specimen material. At lower ultraviolet intensities, as observed in natural or un-accelerated weathering, the rate of oxygen use in bulk photo-oxidation weathering reactions may achieve a balance with the rate of diffusion of oxygen from the ambient air volume into the bulk volume of test specimen material.
Accelerated weathering test apparatus, however, may produce very high UV intensities. As a result, oxygen diffusion from ambient air volume may not occur at a rate fast enough to supply oxygen for photo-oxidation reaction rates may not increase even when the UV intensity is increased many times, since the oxygen concentration in the bulk may be limited by diffusion.
ONATW As, however, produce very high ultraviolet intensities. As a result, oxygen diffusion from the ambient air volume may not occur at a rate fast enough to supply oxygen for photo-oxidation reactions in the bulk of test specimen materials. Consequently, bulk photo-oxidation reaction rates may not increase even when the ultraviolet intensity is increased many times, since the oxygen concentration in the bulk is limited by diffusion rates.
A conventional indoor artificial accelerated weathering test apparatus (“IAAWTA”) which exposes test specimens to selected atmospheres at superambient pressures is disclosed in U.S. Pat. No. 3,664,188, issued to Kockott. This reference includes a test container which is radiation permeable, yet sealed for filling with a suitable atmosphere at a selected pressure. Kockott asserts that the small test container which holds a test speciment is easier to seal. Thus, the atmosphere the specimens are exposed to may be more easily controlled. Alternatively, in apparatus prior to Kockott, the whole test chamber, including the radiation source and all the specimen carriers, is sealed to permit introduction of special atmospheres.
There are several disadvantages of the outdoor natural and indoor artificial accelerated weathering test apparatus discussed above. One disadvantage is that no prior art ONAWTA, such as Robbins, disclose or teach the concept of increasing the presence and/or pressure of oxygen or other fluids other than water around the test specimens. The old testing method simply exposed the test specimens to high ultraviolet exposure under ambient conditions of oxygen. Accordingly, the diffusion rate of oxygen into the test specimen under ambient conditions was limited. Therefore, no matter how accelerated the dose of ultraviolet irradiation, the degradation rate was limited by the diffusion rate of oxygen into the material at ambient concentrations of oxygen in the ambient atmosphere surrounding the test specimen.
Further, the IAAWTA does not teach, suggest or provide motivation to use the inventions disclosed therein in connection with a ONAWTA. One reason such teaching or suggestion is absent is that such IAAWTA fail to duplicate the actual light spectrum of natural sunlight to which the test specimens will actually be exposed to in everyday use and the intensity of natural accelerated weathering apparatus.
It has been acknowledged and recognized by those of skill in the art that the outdoor natural solar light source and indoor artificial light source test apparatus are distinct from one another and provide different sets of empirical data. For example, the Society of Automotive Engineers (“SAE”) has issued Standard Test Method J1961 for accelerated exposure of automotive exterior materials using a solar fresnel-reflective apparatus (outdoor natural solar light source) and Standard Test Method J1960 for accelerated exposure of automotive exterior materials using a controlled irradiance water-cooled Xenon arc apparatus (indoor, artificial light source).
Yet another disadvantage of the INAAWTA is that the spectral power distribution of the ONAWTA is difficult to reproduce. The intensity of the IAAWTA can be adjusted by increasing power. However, the spectral power distribution is substantially different than the sun. Thus, the wavelengths present from the INAAWTA are magnified by the increase in power. As a result, the unnatural degradation mechanisms associated with such wavelengths are also magnified. Therefore, the ONAWTA often provides a more realistic set of data results than the artificial accelerated weathering apparatus because of the closer match to the solar spectrum power distribution found in a test specimen's end use environment.
Therefore, there exists a need in the art for an improved natural accelerated weathering apparatus having a sealed weathering chamber for providing increased rates of photo-oxidation and cooling for the test specimen.